All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae), and currant-lettuce aphid (Nasonovia ribisnigri). These diseases result in millions of dollars of lost lettuce crop throughout the world every year.
Downy mildew (Bremia lactucae) is highly destructive of lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, of which various strains or races exist.
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribisnigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as deep in the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers.
Although several known lettuce cultivars exhibit resistance against disease, irrespective of lettuce type, many lettuce cultivars affected produce large leaves that, when cut to smaller size pieces generally result in a lot of cut surface resulting in a diminished shelf life with respect to wound-induced discolouration of these cut surfaces.
At present the attractive red colour in pre-packed lettuce mixtures is often provided by plant species other than Lactuca sativa. Use is made of radicchio rosso (Cichorium intybus), red cabbage or red-veined spinach or red-veined chard. The disadvantage of using leaves from other vegetables than lettuce is that the different taste of these other vegetables is often experienced as undesirable. Radicchio rosso is for example a Cichorium intybus that has a bitter taste. For taste red-leaved lettuce (Lactuca sativa) is usually preferred over other red-leaved species.
The red leaf colour of lettuce is caused by anthocyanin in the leaves. Antocyanin is an antioxidant, which contributes to the healthy image of lettuce. The disadvantage of anthocyanin is the growth reduction due to absorption of light entering the leaf. This absorbed light is not available anymore for photosynthesis and therefore growth is reduced. As lettuce producers are looking for high yielding varieties, i.e. growth, medium red lettuce varieties are often considered as a good compromise between yield and an attractive red leaf colour.
Although several known lettuce cultivars can be harvested mechanically at young plant, i.e. babyleaf stage, no pest and disease resistant lettuce cultivars exist that can be harvested mechanically at mature stage and still provide leaf pieces that are of small, directly edible size and have an attractive medium red colour. Mechanical harvesting saves labour cost and improves labour conditions in comparison with commonly applied hand-harvesting methods.
For mechanical harvesting it is also important to have a slow bolting variety with a short core. The horizontal knife should cut through the leaves, but not through the core. The lower the core, the lower the knife, and the higher the yield.
There exists a need, therefore, for an mechanically harvestable, medium red, crisp-leaved lettuce variety which exhibits a combination of resistance against downy mildew (Bremia lactucae), and currant-lettuce aphid (Nasonovia ribisnigri). Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.